1. Technical Field
The present invention relates to transmission lines and more particularly to systems and methods for adaptive transmission lines that provide low distortion without sacrificing high transmission rates.
2. Description of the Related Art
Very high-speed optical modulators (e.g., greater than 20 Gb/s) are usually implemented as traveling wave (TW) structures. The TW configuration attempts to distribute capacitance of a long modulator such that it presents a reasonable characteristic impedance to a driver circuit instead of a large lumped capacitance. The TW configuration attempts to match the velocity of a microwave modulation signal traveling along an electrode with an optical wave being modulated which is also moving along the electrode.
An ideal, lossless transmission line is designed to have a characteristic impedance (Z0U) and microwave velocity (vpU) that are both higher than final loaded design targets (Z0L, vpL) when the transmission line is unloaded. In this context, unloaded means that the transmission line is not in contact with the waveguide that includes the optical signal. The ideal, lossless transmission line equations for an unloaded condition include:
            Z              0        ⁢                                  ⁢        U              =                  L                  C          U                      ,            v      pU        =          1                        LC          U                      ,            n      pU        =                  c                  v          p                    =              c        ⁢                              LC            U                              
The ideal, lossless transmission line equations for a loaded condition include:
            Z              0        ⁢                                  ⁢        L              =                  L                              C            U                    +                      C            L                                ,            v      pL        =          1                        L          (                                    C              U                        +                          C              L                                            ,            n      pL        =                  c                  v          p                    =              c        ⁢                              L            (                                          C                U                            +                              C                L                                                        where Z is the impedance, v is the velocity, L is the inductance, C is the capacitance, c is the speed of light and n is an index of refraction.
A loading factor is a ratio of a total length of the loading contact elements to a total modulator length. Each of the active loading elements adds capacitance, which lowers a characteristic impedance and microwave velocity. It is difficult to maximize the impedance and velocity of the unloaded transmission lines. Active optical waveguides in typical semiconductor optical modulators have a high capacitance per unit length. Low unloaded characteristic impedance and velocity combined with highly capacitive loading elements translates into very low loading factors (10-50%). This makes it difficult to achieve a final, loaded characteristic impedance of the TW modulator that is close to 50-ohms while providing a reasonable velocity match to the optical signal. A low loading factor means that the modulator is not space efficient, i.e., most of the modulator physical length does not contribute to the optical modulation.
Furthermore, passive portions of the modulator that do not contribute to modulation add optical loss. Further, a microwave modulation signal incurs extra ohmic losses when it has to travel along longer electrodes. A standard problem with lossy transmission lines is that the impedance of the line is a function of frequency, and this results in distortion.